The present invention relates to tomograph acquisition apparatus usable, in particular, in the medical field. It relates essentially to acquiring tomographs with scintigraphy detectors that are easy to use.
The principles of scintigraphy detectors is known in nuclear medicine. They are as follows. A radioactive marker, generally technetium, is injected into a patient. As a function of its nature, the marker is distributed from its point of injection into various portions of the patient's body. In the patient's body, the marker is to be found in the organ under investigation and it reveals the function thereof. The marker produces gamma photons. After passing through a collimator, the gamma photons are detected by a scintillator crystal, whence the term "scintigraphy detector". The crystal transforms the gamma photons into light photons. The light photons are in turn detected by photomultiplier tubes placed looking at the scintillator. The currents that flow through the photomultiplier tubes are in the form of pulses and are a function of the magnitude of the scintillation produced. These currents are applied to a resistance matrix. The resistance matrix outputs "locating" pulses that specify the position at which scintillation took place facing the photomultiplier tubes.
A counter unit connected to the output from the resistance matrix serves to sum the number of such pulses occurring at each location on the scintillator. It is then possible to create an image representative of the activity of the marker in the body by attributing brightness to each image point as a function of the number of strikes that have been counted for each of said locations. Such a method is known in the state of the art under the name "Anger" method. With very fast scintigraphy cameras, e.g. capable of counting up to 200,000 strikes per second, an image constituting a projection of a portion of the human body can be generated in about 30 seconds.
The detector is mounted in a rotary assembly called a gamma camera which also serves to aim the detector. If the detector is aimed in different directions relative to the body, multiple images can be acquired under the same conditions. By acquiring a sufficient number of images for different aiming directions of the detector, the set of image signals can be subjected to processing suitable for obtaining tomographs of the body by algorithmic reconstruction. Given that the accuracy of such tomographic images increases with the number of projection images, it can be seen that such a method leads to periods of examination that are relatively long.
Proposals have already been made to remedy this problem by constructing gamma cameras provided with two, three, or even more detectors. Under such circumstances, the duration of an examination is reduced, substantially pro rata the number of detectors.
However, another problem arises. To obtain projection images, and consequently tomographs, that have very good resolution, it is necessary for the detectors to be placed as close as possible to the body. Unfortunately, patients to be examined are not all the same size, some are fat, others are thin. In addition, depending on the examination being performed, it may be necessary for the detectors to be at various different distances from the body. For example, an examination around the belly requires the detectors to be at a different distance from the patient than an examination around the head, since the diameter of the head is smaller. A known way of solving this problem is to mount the detectors on telescopic arms and to move the detectors initially as close as possible to the patient. During an examination, the detector travels around a circle whose diameter depends on said distance from the patient. A device of this kind is for example depicted in U.S. Pat. No. 4,368,389.
The drawback of such a telescopic mechanism is that it is twice as complex when there are two detectors instead of only one, and so on. The telescopic mechanism is itself mounted on a rotary assembly enabling the detectors to be pointed in different image-taking directions.